class FlxTimer

A simple timer class, leveraging the new plugins system.
Can be used with callbacks or by polling the finished flag.
Not intended to be added to a game state or group; the timer manager
is responsible for actually calling update(), not the user.

Called by the timer manager plugin to update the timer.
If time runs out, the loop counter is advanced, the timer reset, and the callback called if it exists.
If the timer runs out of loops, then the timer calls cancel().
However, callbacks are called AFTER cancel() is called.